1. Field of the Invention
The present invention relates generally to a dual mode write driver providing both a voltage mode of operation and a current mode of operation, and more particularly pertains to a dual mode write driver which combines the required topology in a write driver circuit to allow either current mode operation or voltage mode operation. A logic control signal is applied to an input to the dual mode write driver to cause the circuit to configure for either voltage mode operation or current mode operation. The dual mode write driver is typically used in a tape or disk drive system to write data onto a tape or disk recording medium.
2. Discussion of the Prior Art
FIG. 1 shows a simple voltage mode write driver. For this write driver, the current through the write head is determined by the value of the resistors R1, R2, and the voltage source V1. This assumes that the NFET devices N1, N2 and the PFET devices P3, P4 have a low voltage drop from source to drain when they are turned on.
During a tape or disk write operation, one input IN is applied to the gates of N1 and P3, and a second complementary input NOT_IN is applied to the gates of N2 and P4. Each of the devices N1, N2, P3 and P4 is individually controlled to provide flexibility for operation of the write driver.
In FIG. 1, the devices P3 and P4 are PMOS FET (PFET) devices, while the devices N1 and N2 are NMOS FET (NFET) devices, with FIG. 1 illustrating the connections of each device to its source s, drain d, gate g and also to the substrate Sb.
The voltage mode write driver has a first PFET P3 coupled to a first NFET N1 at a first node which is connected through resistor R1 to a first side of a write head L1 of the write driver, and a second PFET P4 coupled to a second NFET N2 at a second node which is connected through resistor R2 to a second side of the write head L1.
For a normal write operation, the inputs to N1 and P3 are opposite to the inputs to N2 and P4. For example, if the inputs to N1 and P3 are at a zero voltage potential, while the inputs to N2 and P4 are at a potential V1, then the FET devices P3 and N2 are on and have a very low resistance, while the FET devices P4 and N1 are off and have a very high resistance. This causes the write current to flow in the direction from V1 through P3, L1, and N2. Conversely, if the inputs to N1 and P3 are at a voltage level of V1 while the inputs to N2 and P4 are at a zero voltage potential, then the devices N1/P4 are on and the devices N2/P3 are off. This causes the write current to flow in the direction from V1 through P4, L1, and N1.
If the devices P3 and P4 are at a V1 voltage potential, and the devices N1 and N2 are at a zero voltage potential, then all four FET devices are off.
FIG. 2 shows a simple current mode write driver. For this write driver, the current through the write head is determined by the value of the current source I1. Note that no resistors are in series with the write head L1. The circuit of FIG. 2 has many components coupled in a manner similar to, and which function in a manner similar to, the circuit of FIG. 1, such that those functions need not be repeated here.